The invention relates generally to photovoltaic cells and, more particularly, to methods for forming back contact electrodes for photovoltaic cells.
Solar energy is abundant in many parts of the world year round. Thus, photovoltaic (PV) devices, which convert solar energy into electrical energy, have the potential to provide a reliable form of clean, renewable energy in many parts of the world. Typically, in its basic form, a PV (or solar) cell includes a semiconductor junction made of two or three layers that are disposed on a substrate layer, and two contacts (electrically conductive layers) for passing electrical energy in the form of electrical current to an external circuit. Moreover, additional layers are often employed to enhance the conversion efficiency of the PV device.
There are a variety of candidate material systems for PV cells, each of which has certain advantages and disadvantages. Cadmium telluride (CdTe) is a prominent polycrystalline thin-film material, with a nearly ideal bandgap of about 1.45-1.5 electron volts. CdTe also has a very high absorptivity, and films of CdTe can be manufactured using low-cost techniques. In theory, solar cell efficiencies in excess of twenty percent (20%) could be achieved for cadmium sulfide (CdS)/CdTe devices, provided various issues with the quality of the individual semiconductor layers and with the back contact electrode can be overcome.
Because of the high work function of CdTe, conventional metal back contacts are not generally viewed as being suitable. Instead, graphite pastes (either undoped or doped, for example with copper or mercury) are widely used as a back contact for CdTe PV cells. However, these graphite-paste back contacts tend to degrade significantly over time, as can be shown via accelerated lifetime testing. This degradation typically manifests itself as a decrease in fill factor (FF) and/or open circuit voltage VOC over time. The fill factor degradation is typically driven by a decrease in shunt resistance (Rsh) and an increase in the series resistance (ROC) over time. The degradation of the back contact electrodes undesirably leads to degradation of the solar cell efficiency, on a long-term basis.
To date, the failure to develop low-resistance contacts has hindered the commercialization of CdTe solar cells. A cost-effective solution to this problem would remove one of the remaining hurdles for commercializing CdTe photovoltaic modules.
It would therefore be desirable to provide a back contact electrode for a CdTe PV cell, which exhibits less degradation over the lifetime of the PV cell. It would further be desirable to provide an economical method for forming the improved back contact electrode, in order to facilitate commercialization of CdTe PV cells.